Method of producing steel strip of uniform thickness by direct casting



Oct. l0, 1967 E. A. MIZIKAR ETAL 3,345,738

METHOD OF' PRODUCING STEEL STRIP OF UNIFORM THICKNESS BY DIRECT COATING Filed Nov. l0, 1964 2 Sheets-Sheet l FIG. 2c

mOONI slam E@ INVENTORS.

their ATTORNEY OC- 10, 1967 1 E. A. MIZIKAR ETAL 3,345,738

METHOD OF PRODUCING STEEL STRIP OF UNIFORM THICKNESS BY DIRECT COATING Filed Nov. 1o, 1964 2 sheets-sheet 2 FIG. 3e

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INVENTORS. EUGENE AIMIZIKAR wlToLD M.woJc1K a V KUN Ll their ATTORNEY United States Patent O METHOD OF PRODUCING STEEL STRIP OF UNIFORM THICKNESS BY DIRECT CASTING Eugne A. Mizikar, Witold M. Wojcik, and Kun Li, Pittsburgh, Pa., assignors to Jones & Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania i Filed Nov. 10, 1964, Ser. No. 410,066 9 Claims. (Cl. 29--528) ABSTRACT oF THE DISCLOSURE This invention is directed to a method of producing steel strip of uniform thickness by direct casting so that the strip can withstand rolling to the desired gauge. A uniform thickness is achieved by impressing on the skin of the strip as it is formed a pattern of indentations defining a regular network liner in mesh than the random hill and hollow network which the skin solidifies into in the absence of such impressed indentations. The strip as a result of being uniformly thick will be able to withstand rolling without breaking.

This invention relates to the casting of metal strip. It is more particularly concerned With the casting of low carbon steel strip suitable for subsequent rolling and is described hereinafter with particular reference to that material.

Steel strip is conventionally made by casting molten steel of the desired composition into ingots and then reducing the ingots by successive hot rollings. `A large tonnage of steel strip is cold rolled to final gauge, but the starting material for this cold rolling is hot band, s called, which is strip hot rolled to an intermediate gauge. Attempts have been made to eliminate the casting of the steel into ingots by continuously casting the steel through a mold. Attempts have also been made to eliminate both the casting of the steel into ingots and some of the rolling by casting the steel directly into a pair of rolls or moving belts, to produce a cast product requiring little further reduction. While these attempts have had some degree of success with bar shapes, they have not resulted ina commercial process for the production of strip of any considerable width. A seemingly attractive process for the direct production of Wide thin strip is that of solidifying the steel against a chill, that is, a surface much colder than the molten steel, and stripping or removing it therefrom. Suchy processes are exemplified in Wagner Patent 1,025,848 and Sen'dzimir Patent 2,074,812, both of which produce strip by rotating the chill surface in a pool of molten metal. A difficulty with previously known processes of thisV type is that if steel strip is cast thin enough to be easily removed from the chill, it is not Iat all uniform in thickness and breaks up in subsequent rolling.

' It is an object of our invention to provide a process of casting thin steel strip sufficiently uniform in thickness to withstand rolling to the desired gauge. It is another objectto provide a process of casting steel strip of minimum surface irregularity. It is another object of our invention to provide a process of casting steel strip having a controlled surface pattern. Other objects of our invention will appear in the following description of an embodiment thereof presently preferred by us.

Statedbriefly, our process comprises bringing a chill surface into contact with molten steel so that a thin skin of steel solidiiies thereon and creating in that skin a surface pattern of distributed point indentations. The solidified steel is then stripped from the chill surface and suby 3,345,738 Patented Oct. 10, 1967 sequently hot rolled to the desired gauge. To produce cast strip continuously, it is necessary to move the chill with respect to the molten steel and we provide this movement in conventional ways, such as by forming the chill into a cylinder rotating about its axis with a portion of its surface dipping into the molten steel. Conventionally the chill is made of copper, Iand is uid cooled.

Certain aspects of our invention will be more easily understood by reference to the attached figures. FIG- URE l-a is the outer surface, not-in contact with the chill, of low carbon steel solidified against a'smooth chill in the manner known to the prior art. FIGURE l-b is the surface of the steel of FIGURE l-a in contact with the chill. FIGURES 2-a through 2-g Iare chill surface patterns with which We have experimented and whichl we mean steel having a carbon content not above about 0.12%, and otherwise of the composition conventional for hot or cold rolled sheet and strip, such as A.I.S.I. grade 1008. The outer surface of such steel is illustrated in FIGURE 1-a. The surface is one of hills 1 1 sepa- -rated by interconnecting hollows 2 2. 'I'he surface of the same steel in contact with the chill is illustrated in FIGURE l-b. It also displays hills 3-3 and hollows 4-4, which correspond generally to the hills and hollows of FIGURE l-a. The surface area within a mesh formed by interconnecting hollows 4-4 may be of the order of a square inch. The difference in strip thickness between hills and hollows is substantial. When this strip is rolled it breaks up along the lines of the hollows because of the unequal stresses which the rolling induces.

It is our view that this undesirable pattern or network of hills and hollows results from the way the steel solidies against the chill. Initially. a thin layer of steel solidifies against the chill and is q-uickly cooled to a low temperature while successive layers of steel solidify on top of it. As these superimposed layers cool, they shrink,

setting up a system of stresses in the cast strip parallel to the chill face. The shrinkage stresses in the metal relatively remote from the chill face tend to bend the cast material away from the chill face wherever conditions permit such bending. One condition permitting such bending is, of course, fracture of the skin. Presumably, such fractures are initiated in inhomogeneities, such as non-metallic inclusions in the steel.

Whatever their cause, random fractures of thev skin arev followed by uplifting of the edges formed by the fractures be the mechanism which leads to the hill Iand hollow pattern on st-rip cast against a smooth chill surface.

We have found that this hill and'hollow network is v suppressed if a pattern of distributed point indentations m-ust be spaced so that the network or grid they define isv is deliberately imposed on the solidifying skin. These point indentations m-ay be connected by line indentations if desired, but a pattern of line indentatons which do n'otI define a network is not effective. The point indentations of liner mesh than the network of hills and hollows which it is desired to'suppress'. Indenting of the solidifyingsurface of the cast steel is most conveniently accomplished by knurling the surface of the chill. It is conceivable that the steel solidies in a thin skin surrounding the points of the knurled surface andsubsequently fractures at or around those points, or that the skin does not actually fracture, but merely yields between those points so as -to relieve the shrinkage stresses. It is also conceivable that the indentations increase the uniformity of heat transfer over the contact area between the -chill and the solidifying strip. If the chill surface is indented in a pattern defining Ia Anetwork smaller in mesh than that of the random hill and -hollow network, the Contact between metal and chill is now provided by the numerous uniformly distributed small c hill a-reas. These small areas form active sites for solidication With each small tarea behaving as a smooth chill. Since the areas of Contact are uniformly and finelydistr'ibuted, t-he hills and hollows on the outer surface of stri-p cast on such a chill surface will be so numerous and close to one another that the strip will appear to be of uniform thickness. While the theories above set out are believed by us to be correct, we do not consider ourselves bound thereby.

` FIGURES 2-a through 2-g hereof represent seven configurations of chill surface we have employed. The surface of FIGURE 2-a is provided with parallel V- grooves 5 5 which do not define a network. This chill surface, like a smooth chill surface, is not effective for our process. The remaining six FIGURES Z-b through 2-g are of knurled surfaces of various types and spacings. FIGURES 2-b, 2-c and 2-d may lbe described as fine, 4medium and coarse knurled surfaces respectively, and comprise `diamond knurls formed by cutting a family of parallel V-grooves 6 6 and crossing them with another family of parallel V-grooves 7 7 so as to form a surface of points 8 8. The reference characters have been applied Ionly to FIGURE Z-d, but it will be understood that the knurls of FIGURES 2-b and 2-c are formed in the same way. Surface 2-e comprises shallow pyramids 9 9 and is formed in the same way las the knurled surfaces, but with wider angle intersecting V-grooves 10-10 land 11 11 respectively. The surfaces of FIGURES Z-b through Z-e form a series in which the spacing between points and the depth of the grooves increase from the relatively small values of FIGURE 2-b to the relatively large values of FIGURE 2-e. The square wafiie surface of FIGURE 2-f is formed by cutting intersecting families of V-grooves'lZ-lZ and 13 13 of diminished groove angles so that the chill surface consists of separate squares 14-14 covering the major yportion of the chill area. The diamond waffle surface of FIGURE 2-g is similar, but is formed with intersecting grooves 15 15 and 16-16 of wider V-angle so that the surface squares 17-17 occupy only a minor portion of the chill area. Table I lists the groove depth, angle ofy groove, and grid density of the surface illustrated in FIGURES 2-a through 2-g.

`FIGURES 3-a through 3-g represent steel strip surfaces which were cast in contact with chills having surface textures of FIGURES 2-a through 2-g respectively. The surface of FIGURE 3-a is not greatly different from that of FIGURE l-b except for the superimposed pattern of ridges 18 18 corresponding to the groove-s 5 5 of FIGURE Z-a. It displays hills 19 19 and hollows 20 20 hills 21 21, although of lesser height, against -a relatively.

level area 22. The strip of FIGURE 3-c, east on the medium knurled surface of FIGURE 2-c, displays only a few faint traces 23-23 of a network pattern while that pattern is effectively suppressed in t-he surfaces of FIG- URES 3 d through 3-g.

It is undesirable to indent the surfaces of the cast strip any more than is necessary because deeper indentations require more rolling of the cast strip to obliterate. We prefer, therefore, to impress on the skin of the cast strip the medium knurl pattern of FIGURE 2-c, or its equivalent. From Table I it may be seen that the groove depth of this pattern on the chill surface is .017 inch and that the grid density is 576 points per square inch.

The effectiveness of the medium knurled chill surface of FIGURE 2-c in improving the outer surface of the strip is shown in FIGURE 4 which illustrates the outer surface of the steel of FIGURE 3-6. It displays only relatively shallow hill-s and hollows 25 25 and 26-26 respectively. This-surface contrasts very favorably with that of FIGURE l-a which is the outer surface of steel cas-t on a smooth chill. The ratio of thickest to thinnest section of the steel of FIGURE 1 is about 4 to 1, far greater than the thickness ratio of 2 to 1 for the steel of FIGURE 4. The steel of FIGURE l could not be rolled without tea-ring, whereas, that of FIGURE 4Vwas rolled without difficulty.

Low carbon steel strip cast against a medium knurled chill of FIGURE 2 c to ya thickness of about 0.3 inch was grit blasted to remove scale, heated to a temperature of 2200 F., soaked at that temperature for 15 minutes, and then given two passes each of about 50% reduction through smooth rolls. The resulting strip of about .090 inch thickness and at a temperature of about 1600 F. was water spray quenched t-o about 1000 F. and allowed to cool in air to room temperature. The strip was .grit blasted and pickled lightly to remove rolling scale and was then cold reduced to about .035 inch. This material in the fully annealed condition exhibited yield strengths of 31,000 to 33,000 p.s.i., ultimate strengths of 46,000 to 48,000 p.s.i., and uniform elongations of about 35%.

It is desirable to cast the steel at` a temperature no higher than is necessary to maintain it in -a molten state. We prefer to cast ordinary low carbon steel at a temperature of about 2800 F. and no higher than 2950 F. Higher temperatures cause steeper temperature gradients in the cast strip and result in less even thickness. Alloy steels should be cast close to their liquidus temperatures.

The most uniform thickness of low carbon steels is obtained when the carbon content of the steel is below about 0.06%. However, up to about 0.14% carbon the surface irregularities are not objectionable.

We claim:

1. In the process of producing directly cast metal strip by bringing molten metal into contact with `a chill .so as to cause rapid solidification of a skin of metal against the chill surface, thereafter withdrawing the solidified steel from the chill and subsequently rolling the steel to reduce its thickness, the improvement comp-rising .impressing on that skin as it is formed a pattern of indentations defining a regular network finer in rnesh than a random hill and hollow network which the skin would solidify into in the absence of the impressed'indentations.

2. The process of claim 1 in which the meshes of the `defined network are substantially equal in size.

3. The process of claim 1 in which the indentations lare point indentations.

4. The process of claim 1 in which theindentations are point indentations uniformly spaced from each other.

S. The process of claim 1 in which the indentations are point indentations and number not more than about 576 iper square inch.

6. The process of claim 1 in which the depth of the pattern is at least about .017 inch.

5 6 7. The process of claim 1 in which the indentations References Cited are point indentations and' atleast some of them are UNITED STATES PATENTS connected together to form 11ne indentatrons.

8. The process of claim 1 in which the strip is reduced 2577'423 12/1951 Ludwlg etal' by hot rolling in an amount suicient to oblitenate the 5 3101533 8/1963 Rossbach 29-528 X impressed indentation pattern 3,145,119 8/ 1964 Laforce et a1 29-528 X 3,147,521 9/1964 Boehm 29-528 X 9. The proces-s of claim 1 in which the strip after it is withdrawn from the chill surface is first reheated to rolll. ing temperature and then `reduced by hot rolling in an JOHN F' CAMPBELL P "mary Exammer `amount suicient to obliterate the impressed indentation lo P. M. COHEN, Assistant Examiner, pattern, 

1. IN THE PROCESS OF PRODUCING CAST METAL STRIP BY BRINGING MOLTEN METAL INTO CONTACT WITH A CHILL SO AS TO CAUSE RAPID SOLIDIFICATION OF A SKIN OF METAL AGAINST THE CHILL SURFACE, THEREAFTER WITHDRAWING THE SOLIDIFIED STEEL FROM THE CHILL AND SUBSEQUENTLY ROLLING THE STEEL TO REDUCE ITS THICKNESS, THE IMPROVEMENT COMPRISING IMPRESSING ON THAT SKIN AS IT IS FORMED A PATTERN OF INDENTATIONS DEFINING A REGULAR NETWORK FINER IN MESH THAN A RANDOM HILL AND HOLLOW NETWORK WHICH THE SKIN WOULD SOLDIFY INTO IN THE ABSENCE OF THE IMPRESSED INDENTATIONS. 